The present invention relates generally to methods for measuring the strength of a metal processing solution, and more particularly to methods for measuring the plating rate of an electroless metal plating solution and the etching rate of a chemical etching solution.
The method of the present invention is applicable generally to two broad classes of chemical solutions widely used throughout industry. The two classes of chemical solutions include electroless metal plating solutions and chemical etching solutions, as further defined below, and are collectively referred to herein as "metal processing solutions." As also used herein, the "strength" of a metal processing solution refers to the plating rate in the case of an electroless metal plating solution and the etching rate in the case of an etching solution.
More specifically, the term "electroless plating solution" refers to a solution which utilizes a catalytic chemical process to selectively reduce and plate a metal species onto the surface of a catalytic substrate immersed in the solution. Examples of electroless plating solutions widely used in industry include electroless nickel plating solutions and electroless copper plating solutions.
Such electroless plating solutions typically include a reducing agent, such as a hypophosphite salt, and a metallic ionic salt of the metal to be plated. Upon immersion of a substrate having a suitable catalytic surface in the solution, the hypophosphite anions are catalytically oxidized at the substrate surface. The metal cations in the solution are concurrently reduced and plated onto the substrate surface. The reaction is visually observable and is evidenced by the evolution of hydrogen gas from the surface of the substrate. The resulting metal plating is amorphous in character and consists primarily of the reduced metal species together with a relatively small amount of phosphorous. A succinct summary of the chemistry of electroless plating solutions may be found in U.S. Pat. No. 3,375,178 to Locke.
When the metal ions and the reducing agent in an electroless plating solution become depleted beyond a certain point, the strength of the plating solution, as measured by its plating rate, is decreased sufficiently so that it is no longer practical economically to continue using the plating solution. It is then necessary to prepare a fresh solution or add makeup components to raise the strength of the spent solution back to a reasonably acceptable level. The minimum acceptable plating rate, or strength, of an electroless plating solution is commonly referred to as the specification limit of the solution.
Determining whether an electroless plating solution has been depleted beyond its specification limit has been a problem in the past. Several types of tests have been used to determine the strength of electroless plating solutions, but none have proven entirely satisfactory. For example, a straightforward plating rate test may be employed to directly measure the actual plating rate of a given production solution. In such a test, a sample of a standard substrate material having a known surface area is immersed in the plating solution for a predetermined period of time, typically on the order of several hours. The sample is weighed before and after plating. On the basis of the observed weight increase and the surface area of the sample, the plating rate is calculated in units of, for example, milligrams of metal per square centimeter per minute. The primary drawbacks of this method are that it is time consuming and subject to relatively large errors because of the often small difference in the weight of the sample before and after plating.
Alternatively, wet chemical and spectrophotometric analytical techniques have been used to determine the concentration of the active components in electroless plating solutions. Such techniques may, for example, be based on a determination of the concentration of either the metal cations or the hypophosphite ions, each of which vary in concentration continuously as the solution is depleted and which therefore may be empirically correlated with the plating rate of the solution. Such techniques are awkward because they are time consuming and usually require that a sample of solution be removed to a chemical laboratory for analysis. Also, analytical chemical techniques are not entirely satisfactory because the actual strength, or plating rate, of an electroless plating solution is dependent upon a number of factors, including the concentrations of hypophosphite anions, metallic cations, pH and temperature, so that any single one of these factors may not always be relied upon exclusively to give an accurate measure of the plating rate of the solution.
The term "chemical etching solutions" is used herein to refer to a rather wide variety of solutions which act upon a metal substrate to oxidize and dissolve metal from the surface of the substrate. Such solutions range from relatively mild cleaning solutions to relatively strong chemical milling solutions. These solutions contain an oxidizing agent for oxidizing the native metal to a water soluble ionic species. Chemical etching solutions are generally also acidic, although some solutions, for example etching solutions used on aluminum or zinc, may be strongly basic.
One example of a chemical etching solution is a copper stripping solution used to make printed circuit boards. The copper coated circuit boards are masked with a resistant coating having a desired pattern and immersed in a copper stripping solution to etch the metallic copper from the unmasked portions of the board. As another example, a chemical milling solution is typically used in a similar manner on a masked metal substrate to selectively form recessed areas in the surface of the substrate. This technique is particularly useful in forming areas of reduced thickness on large metal panels where it would be impractical to use conventional mechanical milling methods. Finally, other examples of chemical etching solutions include conventional etching or stripping solutions used to roughen or clean the surface of a metal part, or to produce various types of decorative textured surfaces on metal workpieces.
The strength of a chemical etching solution is measured by its etching rate, as expressed, for example, in milligrams of metal etched per square centimeter per minute. The etching rate of such a solution depends upon such factors as the concentration of the oxidizing agent in the solution, the pH of the solution, and the accumulated concentration of metal salts in the solution. In practice, routine use of an etching solution results in gradual depletion of the active components in the solution, with a resulting reduction in its etching rate.
Determination of the actual etching rate of a particular solution is difficult and suffers from many of the same drawbacks as described above with respect to electroless plating solutions. For example, the most straightforward method of measuring the etching rate is to measure the weight loss of a metal sample of known surface area when placed in an etching solution for a predetermined period of time. This method is, however, time consuming and is subject to large errors due to the practical problems associated with precisely measuring a relatively small weight change in a relatively heavy piece of metal. For the reasons discussed above, analytical chemical techniques are also relatively awkward, time consuming and imprecise.
It is therefore an object of the present invention to provide a method for determining the relative or actual strength of a metal processing solution, particularly including the plating rate of an electroless plating solution and the etching rate of a chemical etching solution. More particularly, it is an object of the present invention to provide a method for measuring the strength of a metal processing solution which may be performed in situ and which does not in itself affect the strength of the solution. Further, it is an object of the present invention to provide a method for measuring the strength of a metal processing solution which is fast, simple and which requires no particular expertise or training to practice.